1. Field of the Invention
This invention relates generally to a method for producing trimesic acid by the liquid-phase oxidation of mesitylene in a solvent, and more particularly concerns a method for producing purified trimesic acid at a greater yield by a process involving the aforesaid liquid-phase oxidation.
2. Discussion of the Prior Art
Trimesic acid is employed as a monomer in the production of specialty polymers and resins. Trimesic acid is also employed in the preparation of germicides, fungicides, plasticizers and cross-linking agents. Obviously, the presence of impurities in trimesic acid can have a serious adverse effect on the physical or chemical properties or performance characteristics of any formulation containing trimesic acid itself or any polymer formed from trimesic acid. In addition, impurities in trimesic acid can adversely affect polymerization processes to which the trimesic acid is subjected. Such impurities in trimesic acid formed by the catalyzed, liquid-phase oxidation of mesitylene are often organic impurities or byproducts formed during the oxidation and inorganic impurities corresponding to metal components of the catalysts employed in the oxidation or formed therefrom. Such impurities often impart undersirable color characteristics to the trimesic acid and its polymerization products.
Thus, minimization and removal of such impurities from trimesic acid are highly desirable. However, the removal of organic and inorganic impurities from aromatic polycarboxylic acids formed by the catalyzed, liquid-phase oxidation of polyalkyl aromatics is typically very difficult, and the removal technique employed depends on the specific aromatic polycarboxylic acid from which the impurities are to be removed and the specific oxidation conditions and catalyst employed to make it. Furthermore, techniques for purifying aromatic polycarboxylic acids are often relatively time consuming and involve relatively complex reaction schemes. Therefore, it is highly desirable to produce trimesic acid under conditions such that the production of impurities and their incorporation in trimesic acid are minimized and the yield of such higher quality trimesic acid is improved.
Thus far, no one has recognized the combination of the weight ratio of solvent-to-mesitylene in the oxidation step, the temperature at which crude trimesic acid is crystallized and separated from the mother liquor, and washing the separated crude trimesic acid with water as a means to effect these desirable goals. For example, Kimura et al., U.S. Pat. No. 4,051,178, disclose a method for producing terephthalic acid by the liquidphase oxidation of p-xylene in the presence of a cobalt-manganese-bromine catalyst. The only solvent-to-p-xylene volume ratios disclosed are about 3:1, as recited in column 4, lines 28-39 and column 6, lines 16-17. Kimura et al. disclose that, if desired, the terephthalic acid produced may be washed with water or acetic acid.
Kalfadelis et al., U.S. Pat. No. 3,119,860, disclose a method for the oxidation of mesitylene in the presence of a cobalt-manganese-bromine catalyst and the crystallization and recovery of the resulting trimesic acid in which the volume ratio of solvent-to-mesitylene in the oxidation is in the range of 0.3:1 to 3:1 and the crystallized and separated trimesic acid crystals are purified by washing with an acid reaction medium such as substantially anhydrous acetic acid.
Kurtz, U.S. Pat. No. 3,171,856, discloses a process for purifying aromatic carboxylic acids formed by the liquid-phase oxidation of a methyl aromatic compound in the presence of an essential combination of water and a methylenic ketone as a reaction activator. Kurtz lists typical starting methyl aromatic compounds as toluene, mon-xylene, p-xylene or mixtures thereof and discloses that the methyl aromatic compound should be present in an amount of 2 to 20, preferably 8 to 16, percent by weight of the fatty acid solvent--that is, at a solvent-to-methyl aromatic compound weight ratio of 5:1 to 50:1 or preferably 6.3:1 to 12.5:1. The resulting crude aromatic acid is purified by first heating it to 225.degree.-260.degree. C. and at at least autogenous pressure, then cooling it to below 170.degree. C. to crystallize it and then, if desired, recrystallization.
Zimmerschied et al., U.S. Pat. No. 3,354,202, disclose a process for the liquid-phase oxidation of polymethylbenzenes to benzene carboxylic acids such as trimesic acid, in the presence of a catalyst comprising cobalt, manganese and bromine and at a volume ratio of solvent-to-starting material of 2:1 to 5:1.
Meyer et al., U.S. Pat. No. 3,261,846 disclose a method for making trimellitic acid by the liquid-phase oxidation of pseudocumene in a solvent and in the presence of catalyst comprising cobalt, manganese and bromine components. The only weight ratios of solvent-to-pseudocumene disclosed are 3:1 and 4:1 in column 5, lines 15-16 and column 7, lines 21-22. The method of Meyer et al. does not involve the addition of water to the separated solid trimelletic acid product.